Conventionally, technologies for estimating states of charge (SOCs) of electric storage devices have been known (e.g., JP-A-2012-149947). Examples of such technologies include a technology for estimating SOCs of electric storage devices by a combination of an open circuit voltage (OCV) method and a current accumulation method. The OCV method is a method for estimating open circuit voltages while an electric storage device is in a non-charge/discharge state and estimating SOC of the electric storage device based on the estimated open circuit voltages. The current accumulation method is a method for estimating SOCs of electric storage devices based on accumulated currents of charge/discharge currents during charge/discharge of the electric storage devices.
In estimation by the OCV method, an error in SOC estimation increases as elapsed time since charge/discharge is stopped is shorter. In estimation by the current accumulation method, an error in SOC estimation increases as an accumulated current becomes larger. Conventionally, different reference errors are set for estimation by the OCV method and estimation by the current accumulation method so that a total error calculated from an error in the estimation by the OCV method and an error in the estimation by the current accumulation method does not exceed a predefined reference error. For the estimation by the OCV method, a constant reference time is set based on the reference error. An OCV is estimated based on a terminal voltage of an electric storage device measured when time since charge/discharge of the electric storage device is stopped passed a reference time, and the SOC of the electric storage device is estimated based on the estimated OCV.
However, the conventional technology for estimating the SOC using the reference time that is set at constant may cause various problems. For example, if a relatively long period is set for the reference time, the number of times of SOC estimation during charge/discharge decreases, that is, a period in which the SOC estimation is not performed is prolonged. As a result, an error in SOC estimation becomes larger. If a relatively short period is set for the reference time, the number of times of SOC estimation increases. However, with this configuration, an error in SOC estimation is large. Therefore, there have been demands for technologies for properly setting the reference time and estimating SOCs of electric storage devices more accurately than the conventional SOC estimation.